There is growing recognition of the importance of 'place'on individual risk of disease, although most studies fail to capture the dynamic nature of the risk environment and its effect on the sex trade has been understudied. The Mexico/US border region is undergoing profound alterations in the environment in which sex work takes place, providing a 'natural experiment'through which we can explore the relative contributions of individual and structural factors on HIV transmission. The primary goals of this research are to define social, spatial, and physical factors affecting female sex workers (FSWs) in 2 border cities and determine their relevance to HIV epidemiology, drug use, and access to services. Tijuana and Ciudad (Cd.) Juarez have witnessed escalating community-level violence and, for Cd. Juarez, changes in the location and visibility of the 'red light'district. While migration and cross-border interactions are major influences, the recent global economic downturn is dramatically altering migration patterns. Based on the above, the specific aims of this project are to: 1) assess changes in social influences on the sex work risk environment over time in both cities and their effect on risk behaviors, HIV/STI incidence, and access to services;2) determine the locations where FSWs live, work and engage in other activities and their relationship to risk behaviors, perceptions of violence, and access to services;and 3) determine the extent to which the built environment and other sex work venue characteristics relate to individual-level behaviors and HIV/STI incidence. To meet these aims, we will recruit 600 FSWs (300 per city) and collect sociodemographic, location, and behavioral data through interviews. All will be tested for HIV, syphilis, gonorrhea, and Chlamydia and treated as needed. Follow-up interviews and testing will occur at months 6, 12, and 18. To meet aim 2, we will construct a geographic information system (GIS) of both cities and explore factors in relation to where FSWs live, work, buy/use drugs, and access services. Changing spatial relationships, such as the dispersal of the Cd. Juarez Zona Roja and intra-urban or cross-border mobility, will be analyzed to track patterns of infectious disease spread. We will also conduct in-depth interviews and an activity-travel survey with 30 sex workers per city, stratified by geography and venue (e.g. street, bar, etc.) to create geo-narratives based on time-geographic methods and computer-aided qualitative data analysis in order to explore the impact of recent social, economic, political, and other structural changes on participants'lives. To meet aim 3, field measurements of the built environment and other venue characteristics will be combined with individual-level data to explore their effect on health outcomes. The data collected will provide information vital to reframing HIV and drug use interventions to take into account the structural environment. This project is timely as HIV prevalence is rising along the U.S./Mexico border, presenting a window of opportunity to prevent transition to a generalized epidemic.